Histidine brace copper proteins (lytic polysaccharide monooxygenases): structure, oxygen activation and biotechnological applications Azza Hassoon 1,2 , Tamás Gajda 1 1 University of Szeged, Hungary, 2 Mansoura University, Egypt
One of the most important industrial challenges for a low fossil fuel economy future is the conversion of biomass into fuels and other useful chemicals. First-generation biofuels are well-established, but their dependence on food source suitable for human consumption means that they should be substituted. Because of its low cost, abundance and renewability, the degradation of lignocellulosic biomass (LB) to produce fuels and other useful chemicals is a very promising perspective. One of the major obstacles of LB valorisation is its surprisingly high recalcitrance to enzymatic hydrolysis caused by the heterogeneous multi-scale structure of plant cell walls. The discovery of lytic polysaccharide monooxygenases (LPMOs) has revolutionized our view on how cellulose is degraded [1]. In contrast to typical cellulases, which are hydrolytic enzymes, LPMOs cleave the β-1,4-glycosidic bonds via the oxidation of the C1- or C4 atom making the substrate tractable to hydrolases, apparently without the need for de-crystallization. In doing so, LPMOs boost the activity of canonical glycoside hydrolases by up to two orders of magnitude, thereby greatly reducing the financial and environmental penalties associated with the use of recalcitrant polysaccharides as a feedstock. This placed LPMOs at the centre of biochemical/bioinorganic research. However, our knowledge on the functioning of these enzymes is rather limited, the identity of the reactive species, the key steps in the oxidative mechanism and even the identity of the biologically relevant oxidizing co-substrate (O2 or H2O2), are still unknown. To this end, the goal of this project is to develop an heterogenous catalyst inspired by the catalytic site of the lytic polysacchride monooxygenase (LPMO). The catalytic site of this enzyme is composed of a copper (II) ion bound by two histidine and a solvent molecule. The first histidine is bound by its imidazole side chain and the second one is bound in a clamp like coordination, by the N-terminal amine and an imidazole side chain. In this type of catalytic core called ‘’Histidine Brace’’, the copper possesses a distorted geometry between square planar and tetrahedral which favors the copper redox cycling (Cu(I)/Cu(II)). This system was successfully synthesized and investigated using several spectroscopic tools. Our kinetic results indicate that the copper complexes in the presence of hydrogen peroxide as oxidising co-substrate, possess significant LPMO-like activity both at pH 7.4 and 10.5. Their enzyme-like behaviour was confirmed, as the kinetic data could be described by the Michaelis- Menten kinetics. References 1. Vaaje-Kolstad, G., Westereng, B., Horn, Z. Liu, Zhai, H., Sørlie, M. and Eijsink, V.G.H., Science, 2010, 330, 219–222.
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